This key describes a reference code to a mapillary image.
This image should be representative of the object, and could be used in a map popup or similar.
If there are multiple images, the "best" image should be prioritized.

If the mapper can see the image in Mapillary's site, he just needs to get the code after http://www.mapillary.com/map/im/ from the URL.
For example, the tag mapillary=US-sClDbAWhtGYc-d3L4nw references the image in http://www.mapillary.com/map/im/US-sClDbAWhtGYc-d3L4nw.

The object itself can also be tagged in the Mapillary picture, according to the Mapillary tag list.

If the intention is simply to point to Mapillary as the source of data for the OSM object, it's enough to add source=mapillary in the object or changeset.